The present invention relates to AC power supply apparatus and methods, and more particularly, to apparatus and methods for controlling the output of an AC power supply load in load sharing applications.
Uninterruptible power supplies (UPSs) are power conversion devices that are commonly used to provide conditioned, reliable power for computer networks, telecommunications networks, medical equipment and the like. UPSs are widely used with computers and similar computing devices, including but not limited to personal computers, workstations, mini computers, network servers, disk arrays and mainframe computers, to insure that valuable data is not lost and that the device can continue to operate notwithstanding temporary loss of an AC utility source. UPSs typically provide power to such electronic equipment from a secondary source, such as a battery, in the event that a primary alternating current (AC) utility source drops out (blackout) or fails to provide a proper voltage (brownout).
In some UPS applications, it may be desirable to parallel connect the outputs of multiple UPSs to provide increased capacity and/or redundancy. A common difficulty in such parallel redundant operation is achieving desirable load sharing among the parallel-connected UPSs. In particular, parallel connection of UPS units that actively regulate their output voltages may lead to overloading of some units and underloading of others. In addition, currents flowing between parallel-connected UPSs may have undesirable effects. For example, in parallel-connected UPSs that use output inverters that generate AC output voltages from intermediate DC links, currents flowing between the parallel-connected UPSs can lead to undesirable increases in voltages on selected ones of the DC links.
Several approaches for dealing with such load sharing issues have been proposed. For example, U.S. Pat. No. 5,745,356 to Tassitino, Jr. et al. describes several techniques for load sharing control, including a xe2x80x9cdifference from averagexe2x80x9d technique that involves signaling between parallel-connected UPSs, as well as other techniques that do not require such interunit signaling. So-called xe2x80x9cdroopxe2x80x9d techniques for providing load sharing in parallel-connected AC power supply systems are described in xe2x80x9cParallel Operation of Single Phase Inverter Modules With No Control Interconnections,xe2x80x9d by Tuladhar et al., Proceeding of 1997 IEEE Applied Power Electronics Conference, vol. 1, pp. 94-100 (1997), and in xe2x80x9cControl of Parallel Connected Inverters in Standalone AC Supply Systems,xe2x80x9d by Chandorkar et al., IEEE Transactions on Industry Applications, vol. 29, no. 1, January/February 1993.
Although conventional techniques can be effective in achieving desirable load sharing and other characteristics in parallel redundant applications, there is an ongoing need for practical and effective techniques for controlling parallel-connected AC power supplies.
According to some embodiments of the invention, an AC power supply includes an output, a reference signal generator circuit operative to generate a reference signal representative of a desired voltage waveform at a node connected to the output, a power determiner circuit operative to generate an estimate of instantaneous reactive power transferred between the output and the node, and a reference signal compensator circuit responsive to the reference signal generator circuit and to the power determiner circuit and operative to generated a compensated reference signal from the reference signal responsive to the estimate of instantaneous reactive power. The AC power supply further includes an AC voltage generator circuit responsive to the reference signal compensator and operative to transfer current between the output and the node responsive to the compensated reference signal. For example, the AC voltage generator circuit may include a controlled inverter that includes an output voltage control loop that receives the compensated reference signal at a reference input thereof.
In some embodiments of the invention, the power determiner circuit may be operative to generate the estimate of instantaneous reactive power from an estimate of instantaneous current and an estimate of instantaneous voltage at the output of the AC power supply. The reference signal may include a first sinusoidal reference signal having a frequency and phase indicative of a frequency and phase of the desired voltage waveform at the node. The power determiner circuit may include a first multiplier operative to multiply the first sinusoidal reference signal by an estimate of voltage (e.g., RMS voltage) at the output to produce a second sinusoidal reference signal representative of the instantaneous voltage. The power determiner circuit may further include a phase shifter circuit operative to process the second sinusoidal reference signal to produce a third sinusoidal reference signal that is quadrature phase shifted with respect to the second sinusoidal reference signal, and a second multiplier operative to multiply the third sinusoidal reference signal by the estimate of instantaneous current to produce the estimate of instantaneous reactive power.
In further embodiments of the invention, the reference signal compensator circuit is operative to generate the compensated reference signal by time-shifting and/or amplitude modulating the reference signal responsive to the estimate of instantaneous reactive power. For example, in some embodiments the reference signal includes a first periodic signal and the reference signal compensator circuit is operative to amplitude modulate the first periodic signal responsive to the estimate of instantaneous reactive power to generate a second periodic signal. The AC voltage generator circuit is operative to control current transfer between the output and the node responsive to the second periodic signal. In addition to such reactive power compensation, in some embodiments, the power determiner circuit is operative to generate an estimate of instantaneous real power transferred between the output and the node, and the reference signal generator circuit is operative to amplitude modulate the first periodic signal responsive to the estimate of instantaneous reactive power and the estimate of instantaneous real power.
According to other embodiments of the invention, the reference signal compensator circuit is operative to process an RMS voltage reference signal according the estimate of instantaneous reactive power to generate a compensated RMS voltage reference signal. The power supply further includes an RMS voltage determiner circuit operative to generate an RMS voltage signal representative of an RMS voltage at the output, a summing circuit that generates an RMS voltage error signal from the RMS voltage signal and the compensated RMS voltage reference signal, and an RMS voltage compensation circuit operative to process the RMS voltage error signal according to a predetermined compensation to produce an RMS voltage compensation signal. The reference signal compensator circuit is further operative to amplitude modulate the first periodic signal according to the RMS voltage compensation signal to produce the second periodic signal. The reference signal compensator circuit may be further operative to time shift and/or amplitude modulate the first periodic signal responsive to the estimate of instantaneous reactive power to generate a compensated periodic signal and to generate the compensated RMS voltage reference signal responsive to the compensated periodic signal.
In still other embodiments of the invention, the reference signal compensator circuit is operative to generate a first sinusoidal signal from the reference signal and to amplitude modulate the first sinusoidal signal responsive to the estimate of instantaneous reactive power to generate a second sinusoidal signal. The AC voltage generator circuit is operative to control current transfer between the output and the node responsive to the second sinusoidal signal. The reference signal compensator circuit may be further operative to time shift the first sinusoidal signal responsive to the estimate of instantaneous reactive power. The reference signal compensator circuit may also be operative to time shift the first sinusoidal signal responsive to the estimate of instantaneous reactive power and to an estimate of instantaneous real power transferred between the output and the node.
In some embodiments of the invention, the reference signal includes a first index signal. The reference signal compensator circuit includes a first compensation circuit operative to process the estimate of instantaneous reactive power according to a first compensation to generate a first compensation signal. The reference signal compensator circuit further includes a summing circuit operative to modify the first index signal responsive to the first compensation signal factor to produce a second index signal that is time-shifted with respect to the first index signal. A sine lookup table circuit is operative to generate the first sinusoidal signal responsive to the second index signal, and a second compensation circuit is operative to process the estimate of instantaneous reactive power according to a second compensation to produce a second compensation signal. A multiplier circuit is operative to amplitude modulate the first sinusoidal signal responsive to the second compensation signal to produce the second sinusoidal signal.
The reference signal compensator circuit may further include a third compensation circuit operative to process an estimate of instantaneous real power transferred between the output and the node according to a third compensation to produce a third compensation signal and a fourth compensation circuit operative to process the estimate of instantaneous real power according to a fourth compensation to produce a fourth compensation signal. The summing circuit may be operative to modify the first index signal responsive to the first and third compensation signals to produce the second index signal. The multiplier circuit may be operative to amplitude modulate the first sinusoidal signal responsive to the second and fourth compensation signals to generate the second sinusoidal signal.
According to other aspects of the invention, an uninterruptible power supply (UPS) includes a DC voltage generator circuit configured to connect to a primary power source (e.g., an AC utility source) and to an auxiliary power source (e.g., a battery) and operative to produce a DC voltage at a DC bus from primary power source and/or the auxiliary power source. A controlled inverter is coupled to the DC bus and operative to produce an AC voltage at an output thereof from the DC voltage on the DC bus responsive to a signal applied to a reference input of the controlled inverter. The UPS also includes a reference signal generator circuit operative to generate a reference signal representative of a desired voltage waveform at a node connected to the output of the controlled inverter. The UPS also includes a power determiner circuit operative to generate an estimate of instantaneous reactive power transferred between the output of the inverter and the node. The UPS further includes a reference signal compensator circuit responsive to the reference signal generator circuit and to the power determiner circuit and operative to generate a compensated reference signal at the reference input of the controlled inverter from the reference signal responsive to the estimate of instantaneous reactive power.
According to method aspects of the invention, a reference signal representative of a desired voltage waveform at a node connected to an output of an AC power supply is generated. The reference signal is modified responsive to an estimate of instantaneous reactive power transferred between the output and the node to generate a compensated reference signal. Current transfer between the output and the node is controlled responsive to the compensated reference signal. In particular, the reference signal may be modified by time-shifting and/or amplitude modulating the reference signal responsive to the estimate of instantaneous reactive power, which may be generated from an estimate of instantaneous current and an estimate of instantaneous voltage at the output of the AC power supply. The reference signal may also be modified responsive to an estimate of instantaneous real power.
Embodiments of the invention can provide improved power transfer control, particularly in applications in which parallel-connected AC power supplies, such as UPSs, are used to provide power to a load at a common node. In particular, by controlling current transfer to the load based on an estimate of instantaneous power, embodiments of the invention can reduce uneven current sharing and can limit voltage increases at inverter input busses. In addition, because such control can be implemented within a high-speed output voltage control loop, some embodiments of the invention can also account from multiple harmonics.